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Soil-Friendly Weed Management:An Ecological Approach

Research-based Practical Guidance for Organic and

Transitioning Farmers

Presented by OFRF Diana Jerkins, PhD & Mark Schonbeck, PhDProduction of the Soil Health Guides & Webinars is made possible by a grant from the Clarence E. Heller Charitable Foundation

In a survey of more than 1,000 organic farmers conducted by OFRF in 2015:

• 67% cited weed management as a high research priority.

• Second only to soil health (74%).

Download full report at http://ofrf.org/

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The organic farmer’s dilemma:

How to control weeds and protect soil health

Non-use of synthetic herbicides:

• Protects soil life and water resources.

• Allows more flexibility for crop rotation.

Greater dependence on cultivation can:

• Stimulate additional weed germination.

• Burn up soil organic matter, hurt soil life,

increase soil crusting and erosion.

OFRF survey respondents’ weed research priorities include:

• Strategies that integrate soil building practices with NOP allowed weed

control tactics.

• Cost-effective strategies for small scale operations.

• Effects of cultivation on soil health.

Research topics

• Impacts of cover crops, rotations, and other

soil health practices on weeds.

• Role of soil microbes in weed suppression.

• Managing nutrients and soil conditions to

favor crops over weeds.

• Breeding crop cultivars that compete

effectively against weeds.

• Managing weeds in perennial crops.

• Weeds as indicators of non-optimum soil

conditions.

• Crop-livestock integration: grazing as a weed

management tool.

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Research topics cont.

Weeds and climate change

• new weeds

• life cycle changes

• weed control in wet conditions

Organic strategies for certain

problem weedsNutsedge

Canada Thistle

(in broccoli)

Johnsongrass

Palmer

AmaranthLambsquartersField

Bindweed

Why weeds happen

An ecological understanding

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Weeds are pioneer plants

Will weeds emerge?

Yes

No

their role is to protect and restore soil after disturbance

How humans

“make” weeds• Decide what plants are “unwanted.”

• Hold succession back at early stages to

produce desired crops (tillage).

• Create open niches (bare soil).

• Bring exotic plants into the region.

Lambsquarters: weed

or nutritious greens?Johnsongrass Purple Nutsedge

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Weeds and the four NRCS Principles of Soil Health Management

• Keep the soil covered as much as possible.

• Maintain living roots year round.

• Minimize soil disturbance.

o Physical (tillage)

o Chemical (nutrients, pesticides)

o Biological (overgrazing, invasive species)

• Diversify soil biota through plant diversity.

Weeds restore cover and

root biomass, protect and

restore soil after tillage,

and absorb excess

nutrients.

Invasive exotic weeds reduce

diversity by displacing native

plants and disrupting the

indigenous soil food web.

Cropland weeds are pioneer plants that:

• Adapt to frequently disturbed but

fertile soils.

• Germinate in response to light

and other tillage cues.

• Grow and develop rapidly.

• Accumulate and respond to

abundant nutrients.

• Reproduce prolifically by seed, or

by rhizomes, tubers, or other

vegetative means.

Common ragweed

and jimsonweed in

tomato

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Prevent weed problems

Strategies to reduce the needfor cultivation

Step 1 - Know the weeds on your farm

• Life cycle, season

• Preferred conditions

• Germination cues

• Weak points

Broadleaf seedlings up to 1 inch (left) are susceptible to

flame, while flame is less effective on grass seedlings

(right) .

Common purslane can

be shaded out by tall

crops like corn.

Hit nutsedge regrowth at the 3-4 leaf stage to deplete

underground reserves.

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Step 2: Close up that niche!

An idle field is the weeds’

workshop.

Winter rye Tillage radish Buckwheat Triticale + field pea

Pearl millet

Put the weeds

out of work…

grow cover crops!

Step 2: Close up that niche!

Intercropping occupies the whole bed.

Mulching blocks weed seedling

emergence.

Relay planting minimizes bare soil after

harvest.

Strip tillage leaves alleys between rows

covered.

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Step 3: Keep the weeds guessing

• Rotate crops, including:

o Cool and warm season crops

o Deep and shallow-rooted crops

o Tall-erect and low-spreading crops

o Crops with contrasting nutrient input needs

• Vary timing of tillage, planting, and harvest.

• Vary method and depth of tillage.

• Vary weed control tactics.

Rotating annual cropland to

perennial sod such as red clover

(above) interrupts annual weed life

cycles and encourages weed seed

predators.

Step 4: Give crops the edge over weeds

A B C

D Healthy soil and good crop management yield

vigorous crops with fewer weeds. Low tunnels

give early snap beans a head start (A). Sweet

potato forms a solid shading canopy (B).

Vigorous tall cultivars like ‘Tennessee red cob’

corn (C) and the heavy canopy of ‘Danvers’

carrot (D) suppress later-emerging weeds.

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Step 4: Give crops the edge over weeds

Transplanting vigorous starts (A) gives crops a six week head start on weeds. In-row

drip irrigation waters lettuce (B) and tomato (C) while leaving between-row weeds

dry.

A B C

Step 4: Give crops the edge over weeds.

• Avoid over-application of

available N, P, and K,

including organic forms.

• Encourage mycorrhizal

fungi.

• For soybean and other

strong N fixers, maintain

low soluble soil N levels

with higher C:N cover

crops or amendments.

0

50

100

150

200

0.5 X 1.0 X 1.5 X 2.0 X

Weeds

Crops

Compost rate (1.0 = recommended)

Re

lati

ve

gro

wth

Weeds (lambsquarters, ragweed, pigweed,

foxtail) respond to poultry litter compost rates

well above saturation rate for crops (corn, kale).

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Step 5: Draw down the weed seed bank

Early season

cultivation …

can fail to give

season-long

weed control …

if last year’s weeds

were allowed to make

a big “bank deposit”!

Step 5: Draw down the weed seed bank

• Prevent weed seed set:

o Mow, cut, pull, or till larger

weeds as soon as flower buds

appear.

• Prevent / interrupt vegetative

reproduction:

o Know when perennial weeds

form tubers, etc.

• Avoid importing new weed seeds

in:

o Mulch hay, manure, compost

o Poorly cleaned crop seedPurple nutsedge can spread by rhizomes and tubers a few weeks after tillage and long before flower heads appear.

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Baiting the weeds: stale seedbed

Prepare seedbed as if to plant (left). Let weeds emerge (center), then till

shallowly or flame. To rebuild soil health in a stale seedbed, Virginia organic

farmer Charlie Maloney (right) broadcasts cover crop seed, then knocks out

tiny weeds and incorporates seed in one pass with rototiller set to work just

one inch deep.

Step 5: Drawing down the weed seed bank:

Let the cleanup crew do its job

• When a cover crop such

as pearl millet or radish

winterkills, leave

residues on the surface

until spring to let weed

seed feeders like

ground beetles reduce

the weed seed bank.

• Delaying tillage

improves soil health

and can increase yield

of the next crop.

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Soil health and weed management

Management practices to enhance both

Crop rotation

“Crop rotation is probably the most important

integrated weed management tool and should be the

cornerstone of any weed management plan.” (Hooks et al., 2016; U Maryland Extension bulletin).

“Short duration rotation with only corn and soybean

are detrimental to soil health … inclusion of a

perennial like alfalfa … is critical for weed control and

soil maintenance.”(Sheaffer et al., 2007;Final report, ORG project 2002-03806).

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TillTill

Till

Till

This diverse, 4-year vegetable /

cover crop rotation may become weedy because

of predictable late-spring soil disturbance.

Cucumber

Crimson clover Lima bean

Barley + crimson

Pepper

Hairy vetchOkra

Cereal rye

Red clover

overseeded

before harvest Till

No-till

roll-

crimp

Potatoes

Till

This 4-year rotation yields three

vegetable crops and one cereal

grain harvest, and helps disrupt

weed life cycles.

LettuceButternut squash

Triticale + field pea

Winter cereal

(for grain)

Red clover

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Cover cropping for effective

weed control

• Select and manage cover crops for rapid

early growth and canopy closure.

• In multifunctional mixes:

o Combine complementary growth

habits, e.g. grass + legume.

o Include one or more species that will

cover the ground quickly

• Optimize growing conditions:

o Planting date and methods

o Sufficient seeding rate

o Fertility, moisture

Buckwheat, 14 DAP

Cowpea, 37 DAP

Complementary architecture and nitrogen dynamics: Grass + Legume

A cover crop of rye + vetch fills the niche (topleft). Broccoli planted no-till into mowed rye + vetch is vigorous and nearly weed-free (center), while broccoli after rye alone is N-deficient and weedy (right), and vetch alone encouraged pigweed to compete with the crop (lower left).

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Healthy soil favors

crops over weeds

• Poor soil structure or slow drainage can promote emergence of certain weeds, and give them an advantage over crops.

• Good soil structure can reduce numbers of these weeds.

• Healthy, biologically active soil grows vigorous crops that require less NPK input and tolerate or suppress weeds.

Yellow nutsedge came up in

wet, compacted field edge

Pigweed emerged mainly from

compacted soil in the footprints.

Management Intensive Grazing (MIG) builds soil

& reduces weeds

Smooth pigweed: good forage.

Spiny amaranth: pasture weed.

Management intensive grazing:

• Avoids preferential grazing

• Suppresses pasture weeds

• Builds soil health and forage vigor

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Soil health and weed management

A few pitfalls to avoid

Organic no-till and cover crop challenges

Cover crop based organic no-till

builds soil health but yield tradeoffs

often occur when:

• Weed seed banks are large.

• Perennial weeds are present.

• Cooler soil or seed-soil contact problems

delay crop establishment.

Some cover crops become weedy if

allowed to set seed:

• Japanese millet, buckwheat

• Cereal grains

• Vetches, crimson clover

Organic no-till was attempted too

soon after breaking sod.

Perennial grasses readily break

through the rolled-crimped winter

cover crop.

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Cautions with organic inputs

Avoid importing weed seeds in:

• Manure (raw or aged)

• Mulch hay or straw

• Weed-contaminated crop seed

• Compost that did not heat to at

least 140°F.

Avoid over applying compost.

• In the Cornell study, high poultry

litter compost rates stimulated more

weed growth than equivalent

amounts of N (feather meal) or K

(potassium sulfate).

Compost is a valuable soil

amendment for building SOM

and soil life. However, more may not be

better, especially for high analysis

materials such as this food waste / yard

waste compost (~1.3-1-1).

Integrated

weed managementGetting the most weed control

with the least soil damage

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Cultivate strategically

• Cultivate shallowly (<1 in) when

weeds are small.

• Use high residue cultivators in

minimum till systems

• Use the best tool for the job,

depending on:

o Crop stage of growth

o Weed species

o Soil conditions, etc.

• See video demos at:

http://articles.extension.org/pages

/61887/weed-management-topics

Use alternatives to cultivation

when practical

• Mowing (between rows of

established crop)

• Flame weeding

• Directed hot water or steam (safer

with mulch than flame)

• Mulching – organic, plastic, weed

mat, biodegradable

• Grazing, weeder geese.

• NOP allowed herbicides (based on

vinegar, fatty acids, or essential

oils)See video demos at:

http://articles.extension.org/pages/61887/we

ed-management-topics

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Solving a problem weed:

Canada thistle• Major problem in organic

reduced till grain rotations.

• Suppressed by sudangrass and

sorghum-sudan.

• Set back by mowing when tall

but not yet flowering.

• Suggested rotation strategy:

Include perennial sod in

rotation, cut for forage.

Plant sorghum-sudangrass

after cool season crop, cut at 3

-5 ft and let regrow.

Canada thistle spreads by seed

and through creeping roots

that send up new shoots.

Solving a problem weed: giant ragweed

• Major problem in soybean phase of 4-

year organic corn-soybean-cereal grain-

forage rotation in Ohio.

• Modified crop rotation added a fifth

year before soybean consisting of:

o Early cultivated fallow (stale

seedbed), then

o Mid-summer buckwheat for grain or

cover crop.

• Substantially reduced ragweed

• Seven year grain and forage rotation

was even more effective.

Weed flush after small grains harvest.

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Small scale solution: opaque tarps

• Laying weed mat or black plastic tarps for 2 – 4 weeks after mowing or roll-crimping cover crops ensures cover crop termination and suppresses weeds.

Image from Brust, 2014: Organic Weed control in No-till Systems (https://extension.umd.edu/sites/default/files/_docs/articles/OrganicWeedControlUsingNo-till_3-2014_0.pdf)

Orchards and vineyards: managing weeds

during tree/vine establishment

• Orchard floor soil health best in living

cover, second best in mulch.

• Weed mat gave best weed control

during crop establishment; organic

mulch second best.

• Tilled or herbicide fallow severely

degrades soils.

• Zipper arrangement of weed mat

facilitates in row compost and

amendments during establishment

years.

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Questions?

Download the Soil Health and Organic Farming Guides at

www.ofrf.org.

This webinar was made possible by a grant from the

Clarence E. Heller Charitable Foundation.